Main Menu

Aerodynamics: Shultz Engineering

There’s been some increased interest in streamlining motorcycles lately, what with Terry Hershner’s bike and now Arthur Kowitz’s fully faired dustbin-streamliner Brammo Empulse, shown below. In spite of the fact that Kowitz claims this is “innovation” it’s not by any definition, but it’s still interesting. The first streamlining I’ve been able to find is via Craig Vetter’s site, the 1937 Joe Petrali Harley Davidson, above.

Honestly, I feel like from a safety standpoint on the racetrack, this is a case of ignoring history at your own peril. Though this is being represented as embracing a neglected technology, the plain truth is that because electric motorcycles are in the dubious position of scrapping for every inch of efficiency, it seems the bargain-with-the-devil solution of a fully enclosed fairing, though not necessarily the outlawed “dustbin”, is worth the risk. Is it? I hope we don’t find out the answer to that with people’s lives. In a recent interview, Hershner said that because his bike is heavy – over 1000lbs – it doesn’t get “tossed around by the wind”. Kowitz, on a test ride approaching 100mph said:

“As we approach(ed) 100mph, the bike gave me a bit of a vague feeling as if it would not be stable as I went even faster… this proved to be untrue, as it never got worse, and I gained confidence in its feel. There were some winds, prevailing and some gustiness, throughout the day. These winds were coming from the northeast, requiring some real muscle to make it turn into the wind. Turn 3 is a high speed left hand sweeper that bends into Sunday’s wind…it took noticeable effort to properly race through turn 3. Another, slower turn gave the same challenge, although to a lesser extent. I felt as thought there were no crosswind challenges..to my pleasant surprise.”

Anecdotal at best, especially in this age of measurable data and wind tunnels. This all leads me to ask exactly what kind of benefit can we expect from these types of fairings, and thus, is it worth some potentially serious, though pretty much unknown risks. So for that, I turn to probably the most authoritative site on streamlining, coincidentally focusing on electric vehicles, Shultz Engineering’s page, here: Kraig Schultz’s Opinions on Aerodynamics and Electric Motorcycles. The site really is just about the last word.

Here’s what I was looking for – a comparison of drag coefficients for various types of fairings over a range of speeds.

The two curves I’m most interested in are the “Modern Racing Motorcycle” and the “Dustbin Fairing”. And they’re not that far apart.

The issue here is that it’s the typical trade-off of Physics. To get the increased efficiency of a fully enclosed fairing you increase your exposed side area – ironically, to a cross-wind, just about the worst shape you could ask for, aerodynamically speaking. Look at it this way – as you increase your efficiency from one direction (the front), you’re sacrificing efficiency from other directions (the sides). So the issue becomes, what are the risks, and are they worth it compared to the savings over a typical, benign-handling sport bike?

For another reference from a legendary source, let’s look at Tony Foale’s site – again, pretty much the last word, but this time on motorcycle handling and frame design:

Aerodynamic design of motorcycles is more than just a matter of producing a low drag, low lift body with a C of P (center of pressure) behind the C of G (center of gravity). Stability is harder to achieve with well streamlined low drag bodies, this is due both to the greater side area present with such fairings and to more efficient production of “sideways lift” due to the angle between the airflow direction and the direction of travel.

So ideally we want a combination of sometimes conflicting requirements:—— Minimal drag for performance and fuel economy. Low frontal C of P. to reduce drag induced weight transfer. Low and rearward side C of P. to reduce the unbalancing moments, and give directional stability. A shape and value of side area that minimizes the side force produced. A high and forward C of G. combined with a large weight to minimize the effect of whatever side forces are generated.

Unfortunately, since the FIM ban on racing with a dustbin in 1957 the technology has been pretty much ignored, so we can’t benefit from any research or development in the face of current (or even ’60s through the ’80s) data-gathering technology. For the record, the ban was largely because of the IOMTT race in 1957. Surtees and MV removed the dustbin due to concerns about crosswinds on the mountain, and subsequently got beaten by a dustbin-faired Gilera ridden by Bob McIntyreon, raising concerns which led to the ban.

However, I have to assume a few things. First, there was a good reason for the ban. At a time when dustbins were at least not uncommon, there was enough concern to take an action like this. There’s no conspiracy theory explanation that holds water, period. So the relative danger of a dustbin, at least under race conditions, is just that – a relative danger. Second, I’ve got to assume that current sportbike technology can balance aerodynamic forward efficiency with safety under a wide range of wind conditions. Of course, they’re also balancing appearance as well, so who knows where the tradeoffs are made – but the numbers I’m seeing above? Hardly worth the risk for the meager payoff of what may be a big risk.

We simply don’t know. But I’m not going to be the one to be on the road with a handful of bike going 100mph who’s going to find out my dustbin doesn’t like a 40mph crosswind.

The bottom line is that I find this discussion disturbing. I have no issue with addressing this honestly, but to simply pass subjective conclusions off as fact, without any discussion of the actual facts of the science involved, seems to me to be particularly irresponsible. …and unfortunately, I feel like we won’t really appreciate the risks until, once again, we see the consequences of ignoring them.

Post navigation

15 responses to “Aerodynamics: Shultz Engineering”

I believe the sportsbike vs. dustbin in the graph is just the dustbin in the front and nothing in the back. There’s a separate line for having both front and back. The back of the vehicle actually has at least as much to do with the aerodynamics as the front. It’s not called drag for nothing. 😉 This is just in response to focusing on the dustbin instead of both.

If the center of pressure is behind the center of gravity the vehicle will be stable. I believe you want C.of P. as close to C. of G. as possible looking from the side. Look at the streamliner world, same as the dustbin. 🙂

Have you read the blogs over at Craig Vetter and Terry Hershner’s sites? I get that crosswind performance is critical, but they are actively solving the problems. Terry’s huge mileage includes plenty of crosswind riding, sometimes in wind that would knock heavy bikes off their stands. TLDR: It’s not unsolvable by any means, just not attempted in recent history.

The basic science is pretty well understood, but you have some pretty complex dynamics going on. A streamliner, with open sides, is harder to model than an idealized shape, in that you can have flows attaching and detaching under different situations. Also, being a two wheeler, roll and yaw interact, and can also affect the aerodynamics. Even if you are Boeing, and have access to wind tunnels, and supercomputers, you can’t be certain what detail you may be overlooking. The simplest way to figure out what works is try stuff. Granted, Boeing engineers are more likely to understand what just happened, and have a better idea of what to do about it. But unless you are the Pentagon, you aren’t likely to get their help.

What doesn’t seem to have been tried is spoilers. They simply spoil the lift without creating much drag. I’m not talking about the absurd things they stick on cars in random places. I mean the real things. Sailplanes use them. Small fences that can be popped up about a 1/3 of the way along the chord that spoils the lift. It seems like that would be the ideal solution to the problem of sidewind lift. It shouldn’t be impossible to have them come out automatically if certain combinations of roll rate and bar pressure combine, or simply have them fixed out all the time.

Hey caoliveiri, I created the chart. I worked on that project for less than free, so you can’t fire me.

The chart was created in April 2007 when there was very little data on the behavior of streamlined of motorcycles available on the internet and even less remembered about what we learned in the 1950’s.

You can download the spreadsheet that created that chart (without units) at http://www.schultzengineering.us/drag.xls . If you visit the source of the chart, you will also read the following about the chart. “Based on these simple assumptions, I should be able to take my ugly, unfaired motorcycle and accomplish the performance gains shown on the chart below (this chart was created mathematically and is not based on actual data) by enclosing it in aerodynamic shapes (note velocity is in MPH):”

As a result of that unit-less chart I went on to build some of the most efficient motorcycles in the world. The shape and divergence of the curves is what’s important.

Since 2007, we collected and posted actual data to support that streamlining does in fact work in the real world. Also, we’ve collected 100,000+ real world miles of streamlined motorcycle experience that indicate cross winds are manageable. Practical experience has shown that the proper set-up will cause a streamlined bike to automatically lean into cross winds without rider input.

Thanks so much for weighing in on this, Kraig. Can you help me out and post some links to the data you reference that you’ve collected since ’07? I was unable to find any reference to cross-winds or cross-wind modeling.

Kraig Schultz’s Delta-11, 11,500 miles so far – still leans appropriately into the wind by itself. A real plus when winds are gusting, changing directions quickly, like happens when you ride into and out of a clump of trees. The Delta-11 steering geometry and reaction to wind gusts was pure “luck”. The front steering is from a Ninja 1998-2007 Generation 3.
Ninja EX250 which has a headtube rake angle = 27degrees.

The Delta-11 Rake was set at 28 degrees to get an extra .25″ of frame clearance with front tire and to hopefully make it a touch more stable at high speed. Trail=4.083″, Offset=1.5″

Chapter 4 in particular was when I had the most concerns with crosswinds as they were very scary to me having ridden the prototype less than 2,000 miles and not knowing what to expect. Over the last 10,000 miles I’ve learned to trust the bike and that it will heel over correctly and just enjoy the ride – I’ve not had enough experience on other forms of bikes to know if this is normal or not. There is one, three lane, expressway, high traffic 100 yard section on a high bridge over a river with metal grating that I don’t like riding over in high/gusty winds. It’s on my daily commute and I try to ride in the wind shadow of car or truck when crossing it. But, the metal grate scares me EVERY time I cross it – like this morning when it was 35 degrees. I just don’t feel comfortable with the traction available on that type of surface.

Alan Smith’s gen 3 Ninja 250 has been in some really good wind storms in his travels back and forth across the USA. I think he’s been across the county and back 4 times so far with streamlining.

Terry Hershner has put over 40,000 on his Zero S streamlining and has some really good stories about riding in winds that had other vehicles worrying.